It is known in the art to use a method for the generation of a sawtooth voltage by a pulse generator for triggering the primary-side switching transistor of stabilized switching power supplies in a control and regulating circuit. Thereupon, the sawtooth voltage is supplied to a pulse duration modulator which varies the duration of the conductive state of the switching transistor and consequently the energy output taken from the supply. In the secondary circuit of the switching power supply a voltage is induced which is rectified and smoothed and which represents the output voltage of the switching power supply. This voltage is compared with an internal reference voltage and, in the event of deviations relative to this reference value, is readjusted by the control and regulating circuit as a function of the duration of the conductive state of the switching transistor.
The sawtooth generator of the switching circuit comprises a current source as well as a transistor which is used for alternately charging and discharging a capacitor. Then voltage pulses in the form of sawteeth occur at the capacitor.
The use of switching power supplies for feeding devices equipped with visual display units requires the synchronization of the switching frequency with the line-deflecting frequency of the visual display unit. In the event of deviations between these two frequencies, due to interference of these two frequencies display interferences such as streaks may occur. Therefore, two modes of operation of the sawtooth pulse generator are used, which controls the generation of switching pulses. In a free-running mode, the generator oscillates at a preset rated frequency. The discharging of the capacitor and consequently the resetting of the voltage increase is triggered by a threshold circuit when exceeding a certain upper voltage value. For the other mode of operation, the sync mode, the frequency of the generator is determined by an external synchronization signal. The activation of the respective mode depends on the presence of the synchronization signal at the synchronization input of the generator. When the synchronization input is not triggered, the free-running mode is activated automatically. But when the synchronization signal of a defined voltage level is supplied to the generator, the sync mode is activated.
In connection with the known circuit described above, the synchronization is carried out by supplying the synchronization signal to the output of the upper threshold circuit, and thus the discharging is activated at a positive signal level of the synchronization signal. One disadvantage of this mode is that only synchronization frequencies which are lower than the rated frequency of the generator are permissible. Higher frequencies cannot be processed by the circuit. This results in the requirement that the rated frequency f.sub.N has to be rated at a value definitely higher than the synchronization frequency f.sub.sync. The frequency difference resulting from the difference between the rated frequency and the synchronization frequency under normal operating conditions has to be a preselected so great that in the event of fluctuations due to temperature influence or aging the condition of f.sub.N &gt;f.sub.sync is always observed. This frequency difference must be overcome when free-running mode is changed to sync mode, and in the secondary circuit of the switching power supply this results in a transient decrease of the output voltage relative to the reference value, this decrease being proportional to the frequency difference. The control and regulating circuit counteracts the voltage decrease by supplying more energy from the main supply by means of a current increase. Thereby, an unduly high current can flow in the switching power supply which thus may be cut off temporarily. The switching power supply has to be dimensioned in terms of its operating characteristics so as to accommodate these unfavorable operating conditions which means that it has to be, in effect, "overdimensioned". This disadvantage could be eliminated by having a frequency difference as low as possible between the rated frequency and the synchronization frequency.
A further disadvantage of the known method for synchronization lies in the fact that reliable operation is only guaranteed with defined voltage levels of the synchronization signal. Thus, the activation of the generator only comes about at a voltage level of 2 volts. In general, also, the portion of the circuit which generate the synchronization signal is fed by the switching power supply, wherein, however, the required voltage levels of the synchronization signal are not available immediately after activation of the device. Thus, it is not possible for the generator to start oscillating. In order to avoid this undefined operational state, it is common practice to cut off the synchronization signal for a short period after switching on the generator until the necessary voltage level is reached. The cutting off of the synchronization signal results in an automatic oscillation start of the generator in the free-running mode which only later changes to the sync mode when a synchronization signal is supplied. For that purpose, a special electronic circuit complicated in design is required, which would otherwise be unnecessary if the generator could process any voltage level at its synchronization input.